This invention relates to voltage regulators and more particularly to a switching voltage regulator which proximates the output of a linear voltage regulator.
A voltage regulator is a device which produces at its output high quality electric power, as for example, a constant voltage source, from a low quality electric power input, specifically the supply or the line. The input may vary in a wide range which is unacceptable to some delicate instruments and equipments. To maintain a constant voltage at the output, the regulator has a network between the input and the output, referred to as the voltage compensation network, and the voltage drop across this network is controlled by the error voltage which is obtained by comparing the output with a reference. The network operates in such a manner that the output stays significantly constant even though the input may vary over a wide range.
The voltage compensation network can be resistive, whereby it is composed of resistors and transistors working in the linear mode. A regulator of this design is conventionally called a linear regulator. A linear regulator has good performance in many respects except its low efficiency in power utilization, because the resistive voltage compensation network dissipates significant amount of power. The power loss in this network is eventually converted into heat, which is a major cause of component failure in many machines, and requires a considerable amount of space and money to remove the heat from the machines. The advance of electronic technology and equipments demands more compact and efficient regulated power supplies and therefore new methods have been investigated for making the regulator more efficient and more compact. Efforts have been made to use a reactive voltage compensation network for the regulator. Such network is a nondissippating network and thus the efficiency of the regulator improves significantly because there is no power loss in the voltage compensation network. A reactive voltage compensation network can be used if the input power is chopped or switched on and off into a sequence of pulses. A regulator of this design is called a switching regulator.
The major problem in designing a switching regulator is to control the sequence of pulses, specifically when to switch the power on and off so that the output is kept constant. The obvious and simplest method to control the sequence is to switch the power on when the output drops below a specified voltage and to switch it off when the output rises above the other specified voltage. This approach was frequently used in early design and also used in integrated circuit TL497 produced by Texas Instruments. In this way, the output will, hopefully, stay in the narrow designed range of the two specified voltages disregarding any other conditions. Although in theory this approach is acceptable, in practice a regulator of this design is inferior in performance.
Recently the pulse width modulation method has been used as a means to control the pulses. The pulse modulation can be carried out using known integrated circuits, such as Motorola's MC3420 which is commercially available for this purpose. In a broad sense all switching regulators are of pulse width modulation. The problem, however, is how to determine the width of the pulses in the presence of various parameters, namely the output, the input, and the load. No matter how the switching regulator is designed, the basic problem is how the width should be affected by these parameters. For example, it can be in the form of a simple function or a complex one, an instant response, or a delayed one, or other variations. Determination of the appropriate control of the pulse width modulation is a significant problem of prior art devices.
In addition to switching control, obtaining appropriate feedback is also a problem with prior art devices. Suppose the output changes suddenly due to the change of some parameters. The regulator has to immediately determine the width of the new pulses to be issued. The pulses of this new width, after certain delay due to the voltage compensation network including a low pass filter, arrive at the output to correct the previous change. If the correction is found too large for the change, then it produces at the output a new change of opposite polarity. Consequently, a new correction of opposite polarity is called for and the regulator is thus ringing or oscillating.
Obviously, a switching regulator of optimal performance should have a proper feedback system and produce pulses of a width just about right for the correction of the change at the output and should determine the pulse width in less than a switching cycle time and with advanced knowledge of what exactly the pulses of that width to be issued will do at the output when they arrive sometime later. This feature cannot be accomplished alone by the present low frequency feedback; it needs a high frequency feedback of some sort.
The output of the switching regulator is actually the average of the pulses over a period of time determined by time constant and the delay factor of the low pass filter. This average, together with the reference, is used to control the generation of pulses in such a manner that the average stays constant despite the variation of other parameters. However, the average is not the only statistical quantity produced by the pulses over that period of time; there are other quantities, for example, the variance. Those other quantities represent some of the undesirable occurrences at the output. If a switching regulator has no special controls over those other quantities, the output may ring or even oscillate at frequencies higher than the frequency determined by the parameters of the low pass filter without changing the average. Therefore a switching regulator of optimal performance should have provisions to control or attenuate these to minimum. Since the low frequency feedback is already used for the control of the average, the control of the other quantities must belong to a high frequency feedback.
Accordingly, the pulse width modulation of the present regulators which produces pulses of a width proportional to the error voltage without considering the other parameters and without advanced knowledge of what the issued pulses will do to the output sometime later, cannot produce the best results.